A particular carrier wave of the differentially biphase-coded type, e.g. one conforming to the well-known Manchester code or the split-phase code, is characterized in that its phase is reversed at the beginning of every cycle in which the transmitted message signal has the logical value "1" whereas the phase remains unchanged when that logical value is "0". With a slightly different code, described in commonly owned U.S. patent application Ser. No. 970,417 filed Dec. 18, 1978 by Ezio Cottatellucci and now U.S. Pat. No. 4,213,891, such a 180.degree. phase shift occurs only upon a change of the basic signal from "0" to "1" or vice versa.
The decoding of such a biphase-coded carrier wave can be performed, for example, by algebraically combining the squared or clipped wave with a replica thereof shifted by half a cycle to produce a ternary wave and sampling the latter midway in every other half-cycle of the original wave. The sampling operation can be timed with the aid of correlation pulses or spikes derived by differentiation from the incoming carrier wave, these spikes occurring invariably in the middle of each cycle while also appearing at irregular intervals--depending upon the logical values of the message bits--at the beginning of a new cycle. The correct sampling times can be determined from the recurrent pattern of the spikes.
As pointed out in another commonly owned U.S. patent application, Ser. No. 61,480 filed July 27, 1979 by Riccardo Caldarella et al, an alternate decoding technique yields correct results with sampling in either the first or the second half of a carrier-wave cycle, except for the fact that distortion experienced by the wave during transmission may make sampling in one half-cycle less error-prone in the presence of phase jitter and therefore more desirable than in the other half-cycle. Thus, it is generally necessary to distinguish between "right" and "wrong" half-cycles for sampling purposes, i.e. to select one of two possible trains of sampling pulses which can be derived from the incoming carrier wave by differentiation.
The solutions to these problems described in the above-identified commonly owned applications, whose disclosures are hereby incorporated by reference into the present application, call for the use of phase detectors designed to shift the sampling pulses by half their recurrence period upon detecting their alignment with the "wrong" half-cycles. Such a switchover, however, can be carried out only after a certain delay during which the phasing of the sampling pulses is determined. In systems where a delay of more than a few milliseconds would be inadmissible during message decoding, therefore, it has already been proposed to let the actual message transmission follow an acquisition interval in which a presynchronization oscillation of half the frequency of the unmodulated carrier wave is sent out. This oscillation, appearing as a square wave after the usual clipping to which the modulated carrier wave is subsequently also subjected, is the equivalent of that carrier wave continuously modulated by a binary "1" under the aforementioned split-pulse code, for example.